Current mode logic (CML) circuits are commonly used as drivers in many wired communication applications, for example, in chip-to-chip interconnections of integrated circuits (ICs). High voltage swing in a conventional CML binary or multi-level driver has to have a proportional voltage drop across a respective load resistance, which can cause two problems. First, a low headroom for the differential pair lowers corresponding output impedance which degrades the driver's return loss. Second, a low headroom for the tail current source causes the tail current to vary with process, voltage, and temperature (PVT) corners that makes the output amplitude poorly defined. Solving these problems by using a higher voltage supply, if available, increases the power consumption significantly.
In a conventional CML driver, assuming a 100-Ohm differential load resistance and a 1-V peak-to-peak differential swing, the tail bias current has to be greater than or equal to 20 mA. This translates into a voltage drop across the load resistors that is greater than or equal 500 mV. As the output starts to swing, each output has to swing down to 250 mV in order to get the desired 1-V overall swing. This leaves very low headroom for the differential pair to have enough output resistance and for the tail current source to have a well-defined output current.
Three techniques are currently used to alleviate the headroom problems, none of which is effective enough to solve the problem. For example, using large low-VT devices for the differential pair, to give some extra voltage headroom for the current source, degrades the output return loss and bandwidth of the driver in addition to loading the pre-driver. Using an active current mirror for the tail current source to improve the current accuracy consumes extra power and large area (e.g., for the compensation cap) and is not effective at high frequencies as it can cause inter-symbol interference (ISI), in addition to not solving the bad return loss problem. Increasing power voltage supply (e.g., to 1.5V) instead of 1V can cause the power consumption to increase by ˜50%, for example, from 20 mW to 30 mW.